The present invention relates in general to fluorescence detection systems and in particular to a fluorescence detection system having a movable excitation/detection module for use with a thermal cycler.
Thermal cyclers are known in the art. Such devices are used in a variety of processes for creation and detection of various molecules of interest, e.g., nucleic acid sequences, in research, medical, and industrial fields. Processes that can be performed with conventional thermal cyclers include but are not limited to amplification of nucleic acids using procedures such as the polymerase chain reaction (PCR). Such amplification processes are used to increase the amount of a target sequence present in a nucleic acid sample.
Numerous techniques for detecting the presence and/or concentration of a target molecule in a sample processed by a thermal cycler are also known. For instance, fluorescent labeling may be used. A fluorescent label (or fluorescent probe) is generally a substance which, when stimulated by an appropriate electromagnetic signal or radiation, absorbs the radiation and emits a signal (usually radiation that is distinguishable, e.g., by wavelength, from the stimulating radiation) that persists while the stimulating radiation is continued, i.e. it fluoresces. Some types of fluorescent probes are generally designed to be active only in the presence of a target molecule (e.g., a specific nucleic acid sequence), so that a fluorescent response from a sample signifies the presence of the target molecule. Other types of fluorescent probes increase their fluorescence in proportion to the quantity of double-stranded DNA present in the reaction. These types of probes are typically used where the amplification reaction is designed to operate only on the target molecule.
Fluorometry involves exposing a sample containing the fluorescent label or probe to stimulating (also called excitation) radiation, such as a light source of appropriate wavelength, thereby exciting the probe and causing fluorescence. The emitted radiation is detected using an appropriate detector, such as a photodiode, photomultiplier, charge-coupled device (CCD), or the like.
Fluorometers for use with fluorescent-labeled samples are known in the art. One type of fluorometer is an optical reader, such as described by Andrews et al. in U.S. Pat. No. 6,043,880. A sample plate containing an array of samples is inserted in the optical reader, which exposes the samples to excitation light and detects the emitted radiation. The usefulness of optical readers is limited by the need to remove the sample plate from the thermal cycler, making it difficult to monitor the progress of amplification.
One improvement integrates the optical reader with a thermal cycler, so that the sample plate may be analyzed without removing it from the thermal cycler or interrupting the PCR process. Examples of such combination devices are described in U.S. Pat. Nos. 5,928,907, 6,015,674, 6,043,880, 6,144,448, 6,337,435, and 6,369,863. Such combination devices are useful in various applications, as described, e.g., in U.S. Pat. Nos. 5,210,015, 5,994,056, 6,140,054, and 6,174,670.
Existing fluorometers suffer from various drawbacks. For instance, in some existing designs, different light sources and detectors are provided for different sample wells in the array. Variations among the light sources and/or detectors lead to variations in the detected fluorescent response from one well to the next. Alternatively, the light source and/or detector may be arranged in optical communication with more than one of the wells, with different optical paths to and/or from each well. Due to the different optical paths, the detected fluorescent response varies from one sample well to the next. To compensate for such variations, the response for each sample well must be individually calibrated. As the number of sample wells in an array increases, this becomes an increasingly time-consuming task, and errors in calibration may introduce significant errors in subsequent measurements.
In addition, existing fluorometers generally are designed such that the light sources and detectors are fixed parts of the instrument. This limits an experimenter's ability to adapt a fluorometer to a different application. For instance, detecting a different fluorescent label generally requires using a different light source and/or detector. Many existing fluorometers make it difficult for an experimenter to reconfigure light sources or detectors, thus limiting the variety of fluorescent labels that may be used.
It is also difficult to perform concurrent measurements of a number of different fluorescent labels that may be present in a sample (or in different samples). As described above, to maximize the data obtained in an assay, experimenters often include multiple fluorescent labeling agents that have different excitation and/or emission wavelengths. Each labeling agent is adapted to bind to a different target sequence, in principle allowing multiple target sequences to be detected in the same sample. Existing fluorometers, however, do not facilitate such multiple-label experiments. Many fluorometers are designed for a single combination of excitation and emission wavelengths. Others provide multiple light sources and detectors to allow detection of multiple labels; however, these configurations often allow only one label to be probed at a time because the excitation wavelength of one label may overlap the emission wavelength of another label; excitation light entering the detector would lead to incorrect results. Probing multiple labels generally cannot be done in parallel, slowing the data collection process.
Therefore, an improved fluorometer for a thermal cycler that overcomes these disadvantages would be desirable.